The variant surface antigen repertoires and the rates these antigens are expressed differ among Giardia isolates. In cloned populations trophozoites with new variant antigens are detected every 12-13 generations in isolate WB and once every 6.5 generations in isolate GS. The ability to express variant antigens is genetically determined since some isolates lack genes encoding the surface antigens of other isolates. The variant surface antigens also differ in their ability to resist digestion with the intestinal enzymes trypsin and chymotrypsin. After exposure of sensitive clones to protease, some trophozoites remain alive and these are stably resistant to these enzymes. These antigens may be important for survival of Giardia in the intestines. Neonatal mice infected with Giardia lamblia undergo antigenic variation. Immunological studies using this model reveal T cell responses to Giardia are limited to the intestine and are not present to the variant antigen. The need for host immune responses in the successful treatment of cestode infections with praziquantel was studied in T. taeniaformis infections in the mice. Humoral factors, e.g., antibody are required for successful therapy in T. taeniaformis and probably T. solium infections as well.